Flutlab neden hata veriyor?

class Bildirimler extends StatefulWidget {
  @override
  _BildirimlerState createState() => _BildirimlerState();
}

class _BildirimlerState extends State<Bildirimler> {
  bool valuefirst = false;
  bool valuesecond = false;
  bool value = false;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(
          backgroundColor: Colors.white,
          leading: IconButton(
              icon: Icon(Icons.arrow_circle_left_outlined, color: Colors.black),
              onPressed: () {
                Navigator.push(context, MaterialPageRoute(builder: (context) => Home()));
              }),
          title: Center(child: Text("Bildirimler", style: TextStyle(color: Colors.black))),
        ),
        body: Container(
            child: Column(
          children: <Widget>[
            Row(
              children: <Widget>[
                SizedBox(
                  width: 10,
                ),
                Text(
                  'Yorumları bildirim olarak gelsin mi',
                  style: TextStyle(fontSize: 17.0),
                ),
                Checkbox(
                  value: this.valuesecond,
                  onChanged: (bool? value) {
                    setState(() {
                      this.valuesecond = value;
                    });
                  },
                ),
              ],
            ),
             Row(
              children: <Widget>[
                SizedBox(
                  width: 10,
                ),
                Text(
                  'Checkbox without Subtitle: ',
                  style: TextStyle(fontSize: 17.0),
                ),
                Checkbox(
                  value: this.valuesecond,
                  onChanged: (bool? value) {
                    setState(() {
                      this.valuesecond = false;
                    });
                  },
                ),
              ],
            ),
          ],
        )),
      ),
    );
  }
}

this.valuesecond = false!;